1. Field Of The Invention
The present invention relates to a polyamide composition. More particularly, the present invention is concerned with a polyamide composition comprising an aromatic ring-containing polyamide resin and a specific amount of an aliphatic polyester, which composition is extremely excellent in molding properties, particularly in mold release characteristics. The polyamide composition may contain additional components which have been used in polyamide compositions to improve properties other than molding properties, such as Izod impact strength and hinge effect properties, so that the effects of these additional components are also exerted in the present polyamide composition. The polyamide composition can advantageously be used as a material for shaped articles, such as a connector and a relay block in various fields, e.g., automobile, electrical and electronic industries.
2. Discussion Of Related Art
Aliphatic polyamides, for example nylon 6 and nylon 66 are now used in large quantities for a wide variety of usages, e.g., for preparing industrial parts, such as parts of automobiles, electrical and electronic devices and machines and also sporting goods and leisure goods, because aliphatic polyamides generally have excellent heat resistance, mechanical strength, toughness and molding properties. However, aliphatic polyamides are susceptible to moisture absorption due to their chemical structure. When aliphatic polyamides absorb moisture, not only are the aliphatic polyamides likely to undergo dimensional change to a large degree but also the stiffness of the aliphatic polymamides becomes low.
For overcoming the above-mentioned disadvantages, a method has been proposed in which a resin having low moisture absorption properties, such as polystyrene, is blended with a polyamide resin (see Japanese Patent Application Laid-open Specification No. 60-195157). However, although the stiffness of the blend obtained by the above-mentioned method is almost not lowered even upon contact with moisture, the blend is not satisfactory in heat resistance and molding properties.
In order to lower the moisture absorption of a polyamide resin, it has been proposed to introduce aromatic rings into a main chain of a polyamide resin (see Japanese Patent Application Laid-open Specification No. 58-120665). Indeed, an aromatic ring-containing polyamide resin prepared by this method exhibits excellent characteristics, such as heat resistance and stiffness, and only exhibits a small dimensional change and a small lowering of stiffness even upon contact with moisture. However, the aromatic ring-containing polyamide resin has a drawback in that when it is subjected to injection molding or compression molding, it is difficult to release the resultant molded article from the molds. For removing such a drawback, it has been attempted to increase the cooling time in the molding operation and also attempted to conduct molding while applying a mold release agent to a mold. However, these proposals are disadvantageous from the viewpoint of operational efficiency. Various other proposals have also been made for improving the molding properties of an aliphatic polyamide. In these proposals, a metal salt of a higher aliphatic carboxylic acid, such as calcium stearate, an aliphatic bisamide, or a higher aliphatic ester is incorporated in aliphatic polyamides (see Japanese Patent- Application Laid-open Specification Nos. 60-32845, 58-157855, 58-157856, 61-188458 and 62-232456). However, these proposals are not effective for improving the molding properties of the above-mentioned aromatic ring-containing polyamide resins.
U.S. Pat. No. 3,803,076 discloses a method for improving the molding properties of an aromatic ring-containing polyamide, in which a lactone having 3 to 9 carbon atoms is incorporated, as a mold release agent, in the polyamide. However, this method cannot provide an aromatic ring-containing polyamide with sufficiently improved molding properties. In addition, this method has a drawback in that since the lactone is liquid at room temperature, silver streaks are likely to appear on the surfaces of molded articles, particularly on the surfaces of portions of molded articles having small thickness, thereby leading to a remarkable lowering in commercial value of the molded articles. Further, the aromatic ring-containing polyamide having the above-mentioned lactone incorporated therein is likely to adhere to a hopper, so that additional measures must be employed for facilitating handling. For example, for incorporating the lactone as a mold release agent to the polyamide composition, it is necessary to introduce the liquid lactone by means of a gear pump or the like during pelletization of the polyamide composition. Therefore, this method is practically disadvantageous.
Heretofore, an aromatic ring-containing polyamide resin having not only excellent molding properties, such as mold release characteristics, but also being free from a lowering in physical properties upon contact with moisture has not yet been realized.